The art of lithographic printing is based on the inumiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water while the image area accepts ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is reproduced.
A generally used type of lithographic printing form precursor has a light sensitive coating applied to an aluminum base support. Negative working lithographic printing form precursors have a radiation sensitive coating which when imagewise exposed to light hardens in the exposed areas. On development the non-exposed areas of the coated composition are removed leaving the image. On the other hand positive working lithographic printing form precursors have a coated composition, which after imagewise exposure to light of an appropriate wavelength, becomes more soluble in the exposed areas than in the non-exposed areas in a developer. This light induced solubility differential is called photosolubilisation. A large number of commercially available positive working printing form precursors coated with quinone diazides together with a phenolic resin work by photosolubilisation to produce an image. In both cases the image area on the printing form itself is ink-receptive or oleophilic and the non-image area or background is water receptive or hydrophilic.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing form precursor with a vacuum to ensure good contact. The printing form precursor is then exposed to a light source, a portion of which is composed of UV radiation. In the instance where a positive printing form precursor is used, the area of the film that corresponds to the image on the printing form precursor is opaque so that no light will strike the printing form precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed.
More recent developments in the field of lithographic printing form precursors have provided radiation-sensitive compositions useful for the preparation of direct laser addressable printing form precursors. Digital imaging information can be used to image the printing form precursor without the need to utilise an imaging master such as a photographic transparency.
An example of a positive working, direct laser addressable printing form precursor is described in U.S. Pat No. 4,708,925, issued Nov. 24th 1987. This patent describes a lithographic printing form precursor in which the imaging layer comprises a phenolic resin and a radiation-sensitive onium salt. As described in the patent, the interaction of the phenolic resin and the onium salt produces an alkali-insoluble composition which is restored to alkali solubility upon photolytic decomposition of the onium salt. The printing form precursor can be utilised as a positive working printing form precursor or as a negative working printing form precursor using additional process steps between exposure and development as detailed in British Patent no. 2,082,339. The printing form precursors described in U.S. Pat. No. 4,708,925 are intrinsically sensitive to UV radiation and can be additionally sensitised to visible and infra-red radiation.
A further example of a laser addressable printing form precursor which can be utilised as a direct positive working system is described in U.S. Pat. No. 5,372,907, issued Dec. 13th 1994, and U.S. Pat. No. 5,491,046, issued Feb. 13th 1996. These two patents describe a radiation induced decomposition of a latent Bronsted acid to increase the solubility of the resin matrix on imagewise exposure. As with the printing form precursor described in U.S. Pat. No. 4,708,925 these systems can be additionally utilised as a negative working system with additional process steps after imaging and pre-development. In the negative working process the decompose by-products are subsequently used to catalyse a cross-linking reaction between resins to insolubilise the imaged areas prior to development. As in U.S. Pat. No. 4,708,925 these printing form precursors are intrinsically sensitive to UV radiation due to the acid generator materials used.
The hereinabove described printing form precursors of the prior art which can be employed as direct imaged positive working printing form precursors are lacking in one or more desirable features. None of the printing form precursors described can be handled extensively without due consideration for the lighting conditions in the working area. In order to handle the printing form precursors for unlimited periods special safelighting conditions are required which prevent unwanted exposure to UV radiation. The printing form precursors may be utilised for limited periods only in white light working conditions dependent on the output spectrum of the white light source. It would be desirable to utilise digital imaging hardware and printing form precursors in the unrestricted, white light press room environment in order to streamline workflows and UV sensitivity would be a disadvantage in these areas. In addition, white light handling would provide an improved working environment in traditional pre-press areas which currently have to be under restrictive safelight conditions.
Moreover, both printing form precursor systems have constraints on their components which create difficulties in optimising plate properties to provide optimum performance across the wide range of demanding lithographic plate performance parameters, including developer solubility, ink receptivity, runlength, adhesion.
In the systems described in U.S. Pat. No. 4,708,925 the presence of functional groups which would crosslink the phenolic resin in the presence of the onium salts upon irradiation cannot be allowed, either as a modification to the alkali soluble resin or as additional components in the composition, as this would lead to reduced solubilisation on exposure.
An essential requirement of the compositions described in U.S. Pat. No. 5,491,046 is the presence of both a resole resin and a novolak resin in order to allow the use of the system in a negative working mode. This is the favoured mode for this system as demonstrated by the negative working patent examples and the first commercialized product derived from this proprietary technology, Kodak's Performer product. This optimisation for negative working potential restricts optimisation for the positive working mode which does not have this requirement.
A wide range of heat solubilising compositions useful as thermographic recording materials have previously been disclosed in GB 1,245,924, issued Sep. 15th 1971, such that the solubility of any given area of the imagable layer in a given solvent can be increased by the heating of the layer by indirect exposure to a short duration high intensity visible light and/or infrared radiation transmitted or reflected from the background areas of a graphic original located in contact with the recording material. The systems described are varied and operate by many different mechanisms and use different developing materials ranging from water to chlorinated organic solvents. Included in the range of compositions disclosed which are aqueous developable are those which comprise a novolak type phenolic resin. The patent suggests that coated films comprising of such resins will show increased solubility on heating. The compositions may contain heat absorbing compounds such as carbon black or Milori Blue (C.I. Pigment Blue 27). These materials additionally colour the images for their use as a recording medium.
The level of solubility differential in the compositions described in GB 1,245,924, however, is very low compared to that of commercial positive working lithographic printing form precursor compositions. Standard lithographic printing form precursors are able to demonstrate excellent tolerance to strong developing solutions, good robustness to variations in customer use and can be optimised to provide high developer solution usage and high numbers of printed impressions. The very poor developer latitude exhibited by the compositions of GB 1,245,924 makes them unsuitable for commercially acceptable lithographic printing form precursors.